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Tropical Storm Nora Forecast Discussion Number 9

2021-08-27 22:49:02| National Hurricane Center (East Pacific)

Issued at 400 PM CDT Fri Aug 27 2021 000 WTPZ44 KNHC 272048 TCDEP4 Tropical Storm Nora Discussion Number 9 NWS National Hurricane Center Miami FL EP142021 400 PM CDT Fri Aug 27 2021 A 1656 UTC ASCAT-B scatterometer pass showed that Nora still does not have a tight inner core and instead has a 30-35 n mi region of winds 20 kt or less. However, the pass did show maximum wind vectors a little over 50 kt, so the storm has been strengthening, and maximum winds are estimated to be 55 kt. This is supported by Dvorak estimates of T3.5/55 kt from TAFB and SAB. A slight adjustment was made to Nora's initial position based on the scatterometer data, but the storm is still moving toward the northwest (310/10 kt). Nora is expected to maintain a motion toward the northwest or north-northwest as it heads in the direction of a weakness in the subtropical ridge which has developed over the U.S. southern Rockies and northern Mexico. The track guidance envelope appears to have tightened up compared to the past few days, with nearly all the model trackers showing Nora's center passing offshore just west of Cabo Corrientes, Mexico, in about 24 hours and then continuing northwestward either across the waters of the Gulf of California or inland over Baja California Sur. The new NHC track forecast has been nudged westward from this morning's forecast, but this is mainly due to the adjustment of the storm's initial position. This track is very close to the HCCA and other consensus aids. Nora's convective structure appears to be responding to a decrease in northeasterly shear, which is analyzed to be out of the northeast at 10-15 kt. This shear is forecast to decrease to 10 kt or less by tonight. Although Nora's broad structure could limit the storm's rate of strengthening, warm waters and upper-level divergence should allow it to become a hurricane over the next day or so. After that time, Nora's intensity will be strongly modulated on whether its center moves over mainland Mexico or the Baja California peninsula. If the center remains over water, as is shown in the official forecast, then environmental conditions appear conducive for Nora to remain as a hurricane for several more days, and the NHC forecast still closely follows the ECMWF-based SHIPS and LGEM models. That said, there is still greater-than- normal uncertainty in the intensity forecast after 48 hours. Key Messages: 1. Nora is forecast to strengthen to a hurricane on Saturday while passing nearby the coasts of the Mexican states of Jalisco and Nayarit, and hurricane watches and tropical storm warnings are in effect for portions of that coastline. Interests in these areas should closely monitor the progress of Nora and subsequent updates to the forecast. 2. Heavy rain associated with Nora is expected across coastal sections of the Mexican states of Oaxaca, Guerrero, Michoacan, Colima, and Jalisco, as well as Baja California Sur. Life-threatening flash flooding and mudslides are expected. 3. Nora is forecast to move over the Gulf of California as a hurricane early next week, bringing a risk of wind and rain impacts to portions of the Mexican states of Baja California Sur, Sinaloa, and Sonora. Given the above-average uncertainty in the forecast, it is still too soon to determine the magnitude and location of these potential impacts. FORECAST POSITIONS AND MAX WINDS INIT 27/2100Z 15.5N 104.9W 55 KT 65 MPH 12H 28/0600Z 16.5N 105.6W 60 KT 70 MPH 24H 28/1800Z 18.3N 106.2W 65 KT 75 MPH 36H 29/0600Z 20.3N 107.0W 70 KT 80 MPH 48H 29/1800Z 21.9N 107.8W 75 KT 85 MPH 60H 30/0600Z 22.9N 108.6W 80 KT 90 MPH 72H 30/1800Z 23.9N 109.4W 80 KT 90 MPH 96H 31/1800Z 25.9N 110.9W 70 KT 80 MPH 120H 01/1800Z 28.4N 111.8W 55 KT 65 MPH $$ Forecaster Berg

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Tropical Storm Ida Forecast Discussion Number 5

2021-08-27 16:49:16| National Hurricane Center (Atlantic)

Issued at 1100 AM EDT Fri Aug 27 2021 000 WTNT44 KNHC 271449 TCDAT4 Tropical Storm Ida Discussion Number 5 NWS National Hurricane Center Miami FL AL092021 1100 AM EDT Fri Aug 27 2021 Radar imagery from Grand Cayman and Cuba as well as satellite data continue to show an improvement in Ida's overall structure this morning, with an increase in banding, the development of a small central dense overcast, and more recently an improved inner-core feature. Both the NOAA and Air Force reconnaissance aircraft reported that the pressure has fallen to around 996 mb, and the Air Force plane has measured flight-level and SFMR winds that support an initial intensity of 55 kt. Although there is still some southwesterly shear over Ida, the outflow has begun to expand over the northeastern and southeastern portions of the circulation. The upper-level trough near the Yucatan peninsula that has been imparting the shear over Ida is forecast to weaken and move westward during the next 12 to 24 hours, which should result in a more favorable upper-level wind pattern. This, in combination with warm sea surface temperatures and a moist environment along the forecast track of the storm are expected to result in steady to rapid strengthening. Ida is now foreast to become a hurricane when it is near western Cuba and once it moves over the southeastern Gulf of Mexico a period of rapid strengthening is likely to begin, with the NHC intensity forecast explicitly calling for rapid intensification to major hurricane strength between 24 and 48 hours. The official intensity forecast is on the higher side of the intensity guidance but not quite as high as the slightly more aggressive CTCI, HWRF, and HCCA models. In addition to the increase in strength, the dynamical model guidance indicates that Ida's wind field will grow larger as it moves over the Gulf of Mexico and this is reflected in the NHC wind radii forecast. In summary, there is a higher-than-normal confidence that a significant hurricane will impact a large portion of the northern Gulf coast by late this weekend and early next week. Ida is moving northwestward or 320/13 kt. The track forecast reasoning is unchanged from before. A mid-level ridge over the western Atlantic is forecast to move westward and this should keep Ida on a general northwestward heading during the next 48-60 hours. This track will bring the storm across western Cuba later today, over the southeastern and central Gulf of Mexico on Saturday and Saturday night, to the coast of Louisiana by late Sunday. The track guidance is in remarkably good agreement with very little cross-track spread during the first 60 hours or so of the forecast period. After that time, Ida is forecast to reach the western portion of the ridge, which is expected to cause the storm to slow down and turn northward and then northeastward over the southeastern United States. The NHC track forecast is near or just east of the various consensus aids, in best agreement with the GFS ensemble mean. Although the small spread in the guidance through landfall increases the overall confidence in the track forecast, users are reminded to not focus on the exact details of the forecast track as storm surge, wind, and rainfall impacts will extend far from the center. Key Messages: 1. Life-threatening storm surge and hurricane conditions are expected later today and tonight in portions of western Cuba, including the Isle of Youth, where a Hurricane Warning is in effect. Life-threatening heavy rains, flash flooding and mudslides are expected across Jamaica, the Cayman Islands, and western Cuba, including the Isle of Youth. 2. The risk of life-threatening storm surge inundation is increasing along the coasts of Louisiana, Mississippi, and Alabama. Inundation of 7 to 11 feet above ground level is possible within the area from Morgan City, Louisiana, to Ocean Springs, Mississippi, including Lake Borgne. Interests in these areas should follow any advice given by local officials. 3. Ida is expected to be a dangerous major hurricane when it reaches the northern Gulf Coast on Sunday, and the risk of hurricane-force winds continues to increase, especially along portions of the Louisiana coast, including metropolitan New Orleans. Potentially devastating wind damage could occur where the core of Ida moves onshore. 4. Ida is likely to produce heavy rainfall later Sunday into Monday across the central Gulf Coast from southeast Louisiana to coastal Mississippi and Alabama, as well as the Lower Mississippi Valley, resulting in considerable flash, urban, small stream, and riverine flooding. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 20.7N 82.1W 55 KT 65 MPH 12H 28/0000Z 22.2N 83.7W 65 KT 75 MPH...OVER WESTERN CUBA 24H 28/1200Z 24.0N 85.8W 75 KT 85 MPH...OVER WATER 36H 29/0000Z 25.9N 87.8W 90 KT 105 MPH 48H 29/1200Z 27.6N 89.7W 105 KT 120 MPH 60H 30/0000Z 29.0N 90.8W 105 KT 120 MPH 72H 30/1200Z 30.3N 91.3W 60 KT 70 MPH...INLAND 96H 31/1200Z 33.3N 90.2W 30 KT 35 MPH...INLAND 120H 01/1200Z 35.2N 87.1W 20 KT 25 MPH...POST-TROP/REMNT LOW $$ Forecaster Brown

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Tropical Storm Nora Forecast Discussion Number 8

2021-08-27 16:35:43| National Hurricane Center (East Pacific)

Issued at 1000 AM CDT Fri Aug 27 2021 000 WTPZ44 KNHC 271435 TCDEP4 Tropical Storm Nora Discussion Number 8 NWS National Hurricane Center Miami FL EP142021 1000 AM CDT Fri Aug 27 2021 As was expected, vertical shear appears to be decreasing, with deep convection expanding over Nora's surface center just within the past few hours. Water vapor imagery also suggests an expansion of upper-level outflow to the north of the storm. Since Dvorak data-T numbers are 3.0/45 kt from TAFB and SAB, Nora's intensity is being held at that value for now. Nora has turned toward the northwest (310/10 kt) in response to a weakness in the subtropical ridge that has developed over the U.S. southern Rockies. Little change to this steering pattern is expected during the next several days, and Nora is forecast to move toward the northwest or north-northwest for the entire 5-day forecast period. Compared to the last few days, nearly all the track models now have Nora's center staying just offshore of Cabo Corrientes, Mexico, in about 36 hours. The GFS and HMON model fields still show a landfall in that area, but their interpolated trackers--which have been shifted to begin at our estimated initial position--also stay just offshore. After 36 hours, the track guidance has shifted slightly eastward, and the new NHC track forecast now keeps Nora's center over water up into the Gulf of California from days 2 through 5. That will be a small needle to thread, however, and any future shifts in the models could increase the risk of impacts to either the Baja California peninsula or the northwestern coast of mainland Mexico through the middle of next week. SHIPS model diagnostics indicate that the northeasterly deep-layer shear over Nora has decreased to about 15 kt, and this shear is expected to decrease further to below 10 kt in about 12 hours. Along with warm waters of about 29 degrees Celsius and significant upper-level divergence (at least for the next 36 hours), these conditions should foster strengthening, and Nora is still forecast to become a hurricane on Saturday. The part of the forecast after 48 hours remains uncertain depending on how much Nora interacts with land, but based on the new forecast that keeps the cyclone over water in the Gulf of California (where SSTs are even warmer -- around 31 degrees Celsius), Nora could maintain hurricane intensity through at least day 4. The NHC intensity forecast continues to hedge on the side of the ECMWF-based SHIPS and LGEM models, since most of the other models show Nora's center interacting with land to at least some degree. The new forecast has necessitated an extension of the Tropical Storm Warning northward to San Blas, Mexico, and the issuance of a new Tropical Storm Watch north of there to Mazatlan. Key Messages: 1. Nora is forecast to strengthen to a hurricane on Saturday while passing nearby the coasts of the Mexican states of Colima, Jalisco, and Nayarit, and hurricane watches and tropical storm warnings are in effect for portions of that coastline. Interests in these areas should closely monitor the progress of Nora and subsequent updates to the forecast. 2. Heavy rain associated with Nora is expected across coastal sections of the Mexican states of Oaxaca, Guerrero, Michoacan, Colima, and Jalisco, as well as Baja California Sur. Life-threatening flash flooding and mudslides may occur. 3. Nora is forecast to move over the Gulf of California as a hurricane early next week, bringing a risk of wind and rain impacts to portions of the Mexican states of Baja California Sur, Sinaloa, and Sonora. Given the above-average uncertainty in the forecast, it is still too soon to determine the magnitude and location of these potential impacts. FORECAST POSITIONS AND MAX WINDS INIT 27/1500Z 15.4N 104.3W 45 KT 50 MPH 12H 28/0000Z 16.3N 105.2W 50 KT 60 MPH 24H 28/1200Z 17.9N 105.9W 60 KT 70 MPH 36H 29/0000Z 19.9N 106.5W 65 KT 75 MPH 48H 29/1200Z 21.7N 107.4W 70 KT 80 MPH 60H 30/0000Z 23.1N 108.3W 75 KT 85 MPH 72H 30/1200Z 24.2N 109.1W 75 KT 85 MPH 96H 31/1200Z 26.1N 110.7W 65 KT 75 MPH 120H 01/1200Z 28.4N 111.7W 50 KT 60 MPH $$ Forecaster Berg

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Tropical Storm Nora Forecast Discussion Number 7

2021-08-27 11:00:12| National Hurricane Center (East Pacific)

Issued at 400 AM CDT Fri Aug 27 2021 000 WTPZ44 KNHC 270900 TCDEP4 Tropical Storm Nora Discussion Number 7 NWS National Hurricane Center Miami FL EP142021 400 AM CDT Fri Aug 27 2021 The convective structure of Nora this morning continues to look a bit disheveled, with the majority of the deeper convection still located to the south and west of the mean low-level circulation center due to 15-20 kt of northeasterly vertical wind shear. More recently, however, there are a few deeper convective tops attempting to develop on the northeast side of the broad vortex. A helpful ASCAT-A pass at 0307 UTC revealed that the circulation remains quite elongated, with two distinct mesovorticies, one to the southwest embedded in the convection, and another to the northeast. The current center estimate roughly splits the difference between these two points. The peak wind retrieval from that scatterometer pass was also 43-kt with a few higher rain-contaminated values. Assuming a bit of instrument undersampling, the current intensity is set to 45-kt for this advisory. Nora motion continues to gradually bend rightward, with the latest motion estimate at 300/10 kt. As mentioned in the previous discussion, mid-level ridging to the north of Nora is gradually being eroded by a shortwave trough propagating westward across the Rocky Mountains. This pattern is expected to create a weakness in the ridge that should allow Nora to begin gaining more latitude later today. While the models are in decent agreement with this poleward turn, there still remains some spread on how sharp this turn will be, and whether or not the tropical cyclone will directly impact the southwestern coast of Mexico. The latest deterministic ECMWF and UKMET runs continue to forecast Nora to remain offshore of mainland Mexico, while the GFS and Canadian models explicitly show Nora making landfall in 36-48 hours. While the track guidance envelope has narrowed some, the multi-model consensus aids have changed very little this cycle. Therefore, the latest NHC track forecast is quite similar to the previous one, and lies close to the HCCA consensus aid and GFEX, which is a simple blend of the GFS and ECMWF forecast solutions. This forecast track still brings Nora close to Baja California Sur and the Gulf of California in around 72 hours, though uncertainty by that time frame remains high, related to the degree of land interaction Nora will encounter prior to that point. Nora continues to be impacted by northeasterly vertical wind shear, though this shear is still forecast to subside over the next 12-24 hours. As long as Nora stays far enough offshore, the reduction in shear in combination with warm 28-29 C sea-surface temperatures and ample deep-layer moisture favor strengthening. Therefore, the latest NHC intensity forecast still indicates intensification with Nora forecast to become a Hurricane on Saturday. This intensity forecast remains on the high end of the guidance envelope, especially after 36 hours, because a subset of the guidance aids (HWRF, HMON, GFS) bring Nora inland after this time, substantially disrupting the storm's circulation. However, since the latest NHC forecast track keeps Nora far enough offshore, I have elected to follow closer to the ECMWF and ECMWF ensemble mean intensity aids, which show continued intensification after 36 hours. After 72 hours, weakening is anticipated due to possible land interaction with the mountainous terrain of Baja California Sur. Needless to say, this intensity forecast hinges on the track of Nora remaining offshore of mainland Mexico, and this intensity forecast is also more uncertain than usual, especially after 36 hours. Key Messages: 1. Nora is forecast to strengthen to a hurricane on Saturday while passing nearby the coast of southwestern Mexico, and a hurricane watch and tropical storm warning are in effect for portions of that coastline. Interests along the southwestern coast of Mexico should closely monitor the progress of this system and subsequent updates to the forecast. 2. Heavy rain associated with Tropical Storm Nora is expected across coastal sections of the Mexican states of Oaxaca, Guerrero, Michoacan, Colima, and Jalisco. Life-threatening flash flooding and mudslides may occur. 3. Nora is forecast to be near or over the southern portion of Baja California Sur as a hurricane early next week, bringing a risk of wind and rain impacts to that area. Given the above average uncertainty in the forecast, it is still too soon to determine the magnitude and location of these potential impacts. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 14.6N 103.5W 45 KT 50 MPH 12H 27/1800Z 15.3N 104.4W 50 KT 60 MPH 24H 28/0600Z 16.6N 105.4W 60 KT 70 MPH 36H 28/1800Z 18.5N 106.1W 70 KT 80 MPH 48H 29/0600Z 20.6N 106.9W 75 KT 85 MPH 60H 29/1800Z 22.1N 107.9W 80 KT 90 MPH 72H 30/0600Z 23.2N 108.9W 80 KT 90 MPH...NEAR BAJA CAL SUR 96H 31/0600Z 24.9N 110.4W 60 KT 70 MPH...NEAR BAJA CAL SUR 120H 01/0600Z 27.1N 111.6W 35 KT 40 MPH...NEAR BAJA CAL SUR $$ Forecaster Papin/Beven

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Tropical Storm Ida Forecast Discussion Number 4

2021-08-27 10:57:12| National Hurricane Center (Atlantic)

Issued at 500 AM EDT Fri Aug 27 2021 000 WTNT44 KNHC 270857 TCDAT4 Tropical Storm Ida Discussion Number 4 NWS National Hurricane Center Miami FL AL092021 500 AM EDT Fri Aug 27 2021 Radar data from Grand Cayman and satellite imagery indicate that Ida is becoming better organized just to the north of Grand Cayman. The convection has become more concentrated near the center, and the radar data is showing increasing curved banding. In addition, the pressure at Grand Cayman fell to 1003 mb, showing that the central pressure is lower than observed by the aircraft mission yesterday afternoon. Based on a combination of the Grand Cayman observations and satellite intensity estimates from TAFB and SAB, the initial intensity is increased to 40 kt. Air Force Reserve and NOAA Hurricane Hunter aircraft are scheduled to investigate Ida near 12Z. After the earlier wobbling, Ida seems to have resumed a northwestward motion of 320/13. A mid-level ridge centered off the southeast U.S. coast is expected to shift westward through the weekend. This feature should keep Ida moving on a general northwestward track for the next 2 or 3 days, taking the core of the system over western Cuba this afternoon or evening and then across the southern and central Gulf of Mexico this weekend. Although some model timing differences remain, they are in fairly good agreement that Ida will make landfall in Louisiana late Sunday or early Monday. The new forecast track is shifted a little to the east of the previous forecast through 48 h based on the initial location. However, the 60 and 72 h positions are similar to the previous forecast, and thus there is no significant change to the forecast landfall area in Louisiana at this time. Users are reminded to not focus on the exact forecast track as impacts will extend far from the center and the average 72-hour track error is around 120 miles. There continues to be some southwesterly shear affecting Ida due to the storm's location between an upper-level trough to the west and an upper-level anticyclone to the east. Some decrease in the shear is expected during the next 24 h, and that, combined with a moist environment and warm sea surface temperatures along the forecast track, should allow steady to rapid strengthening until the cyclone makes landfall along the northern Gulf coast. The new intensity forecast is slightly stronger than the previous forecast, and it now calls for Ida to reach a peak intensity of 100 kt before landfall. It should be noted that this intensity is lower than that forecast by the HMON model, the HCCA corrected consensus model, and the Florida State Superensemble. After landfall, Ida should weaken as it moves through the lower Mississippi and Tennessee River valleys. Based on the available guidance, there remains higher-than-normal confidence that a significant hurricane will approach and impact the Gulf coast during the weekend and early next week. Key Messages: 1. Tropical storm conditions are occurring in portions of the Cayman Islands, and they are expected in portions of western Cuba and the Isle of Youth today, where a dangerous storm surge is also possible in areas of onshore flow. Life-threatening heavy rains, flash flooding and mudslides are expected across Jamaica, the Cayman Islands, and western Cuba, including the Isle of Youth. 2. There is an increasing risk of life-threatening storm surge inundation along the coasts of Louisiana, Mississippi, and Alabama, where a Storm Surge Watch is in effect. Interests in these areas should follow any advice given by local officials. 3. There is an increasing risk of dangerous hurricane-force winds beginning Sunday along the portions of the coasts of Louisiana and Mississippi, including metropolitan New Orleans, where a Hurricane Watch is in effect. 4. Ida is also expected to produce heavy rains across the central Gulf Coast from southeast Louisiana to coastal Mississippi, Alabama, as well as the Lower Mississippi Valley starting Sunday into Monday, resulting in considerable flash, urban, small stream, and riverine flooding. FORECAST POSITIONS AND MAX WINDS INIT 27/0900Z 20.0N 81.4W 40 KT 45 MPH 12H 27/1800Z 21.5N 82.9W 50 KT 60 MPH...OVER ISLE OF YOUTH 24H 28/0600Z 23.4N 84.9W 60 KT 70 MPH 36H 28/1800Z 25.3N 86.9W 70 KT 80 MPH 48H 29/0600Z 27.1N 88.8W 85 KT 100 MPH 60H 29/1800Z 28.7N 90.2W 100 KT 115 MPH 72H 30/0600Z 30.1N 91.1W 85 KT 100 MPH...INLAND 96H 31/0600Z 33.0N 90.5W 35 KT 40 MPH...INLAND 120H 01/0600Z 35.0N 88.0W 25 KT 30 MPH...INLAND $$ Forecaster Beven

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